DAMASCUS, Syria — After months of drawn-out negotiations, United Nations experts arrived in Damascus on Sunday to begin their investigation into the purported use of chemical weapons in Syria’s civil war.

The rebels, along with the U.S. and other Western powers, have accused President Bashar Assad’s regime of carrying out the alleged chemical attacks, while the Syrian government and Russia have blamed the opposition. Nearly six months after the weapons of mass destruction were first allegedly employed on the battlefield, definitive proof remains elusive.

The U.N. team that arrived in Damascus on Sunday is tasked with determining whether chemical weapons have been used in the conflict, and if so which ones. But the mission’s mandate does not extend to establishing who was responsible for an attack, which has led some observers to question the overall value of the probe.

The 20-member U.N. delegation, led by Swedish chemical weapons expert Ake Sellstrom, checked into a five-star hotel upon arrival in central Damascus. Plainclothes police officers immediately whisked them away from a crush of reporters and cameraman waiting in the lobby.

U.N. spokesman Martin Nesirky said the team will begin its work today.

The investigators are expected to visit three sites where chemical weapons attacks allegedly occurred: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations that have not been disclosed.

Syria is said to have one of the world’s largest stockpiles of chemical weapons, including mustard gas and the nerve agent sarin. There are concerns that the Assad regime might use them on a large scale, transfer some of them to the Lebanese militant Hezbollah group or that the chemical agents could fall into the hands of al-Qaida-affiliated militants and other extremists among the rebels.